Cellular communications systems include a plurality of radio base stations (RBS) or simply base stations (BS) spaced from each other in order to form plurality of cells which provide radio communication in a geographic area. A mobile transceiver unit, such as a mobile phone (or a User Equipment—UE), communicates via a radio channel with the base station of the cell in which the User Equipment is currently located. Because the User Equipment is mobile the distance between it and the base station (the serving base station) may increase so that the signal becomes too weak to maintain the communication. Without any support from the network the connection would be terminated due to diminishing level of the signal received either by the User Equipment or the base station.
In order to maintain the communication a process called handover is implemented. In this process serving of the User Equipment (i.e. providing radio communication channel) is handed over from one cell to another (the cell can be formed by the same base station or by another base station). In order to maintain the continuity of the connection the handover process is carried out between overlapping cells. For two cells to be classified as overlapping, they must fulfill a set of requirements, the most basic of which is that their respective coverage areas must overlap.
In order to accomplish handover each cell in a Universal Mobile Telecommunications System (UMTS) has a list of overlapping cells, which enables both the intra- and inter-system handover processes. Therefore, the overlapping cells in a specific list may belong to the same network, operating at the same or a different frequency, or they may belong to a different mobile network, owned by another operator, or they may even be part of different system, such as a GSM network.
In the known solutions, the list of overlapping cells is an output of the network planning process. These lists are configured statically in the Radio Network Controllers (RNC), which are entities that control large sets (usually several hundreds) of Radio Base Stations (RBS). RNCs and RBSs are entities of the Universal Terrestrial Radio Access Network (UTRAN). It is envisioned that with further development of the mobile telecommunications networks the RNCs may be eliminated and their functionality divided and distributed over RBSs and other entities of the UTRAN. If the configuration of these lists will continue to be static and a result of a centralized planning process such an approach will restrict the ability of the network to react fast and rapidly reconfigure the overlapping cell lists in case of unexpected events or failures. It must be noted that the process of reconfiguring overlapping cell lists is needed after a failure, even if no coverage problem arises. The new network topology emerging as a result of the failure requires that the RBSs reconfigure their respective overlapping cell list in order to support efficient handover and avoid call drops.
Methods of discovering overlapping cells exist and few of them together with their disadvantages are discussed below.
There are known methods based on topology discovery for ad hoc networks, for example the mobile agents-based solution disclosed in R. Chpudhury, et al., “A distributed mechanism for topology discovery in ad hoc wireless networks using mobile agents,” in Mobile and Ad Hoc Networking and Computing, 2000. MobiHOC. 2000 First Annual Workshop on, 11 Aug. 2000, pp. 145-146 or the topology discovery process involved in proactive routing protocols disclosed in T. Clausen (Ed.), et al., “Optimized link state routing protocol (olsr),” RFC 3626, October 2003, Network Working Group. Those methods assume that two neighbours in the topology are two nodes in range of each other. However, neighbouring base-stations of a cellular network such as UMTS may not be in range of each other.
There are other ways of discovering overlapping cells in UMTS networks. With current technology, an RNC can ask a terminal to perform an intra-frequency code search. In this way, it can discover coverage overlaps between cells, but this procedure is limited to cells operating at the same frequency as the terminal. As a consequence, such a procedure can discover neither UMTS cells working in another frequency, nor inter-system cells (e.g., GSM cells).
Hence, an improved method of discovery of overlapping cells would be advantageous and in particular one that can be implemented in a distributed environment and is fast enough to ensure high quality of handover especially in a situation of a cell failure. Accordingly, the invention seeks to preferably mitigate, alleviate or eliminate one or more of the disadvantages mentioned above singly or in any combination.